CHAPTER V. 



THE INTERFEROMETRY OF VIBRATING SYSTEMS. 



52. Introductory. The high luminosity of the achromatic interferences 

 and the occurrence of but two sharp fringes make it possible to utilize them 

 even in cases when the auxiliary mirrors vibrate. Experiments of a similar 

 kind have been previously tried with telephones and the spectrum ellipses; 1 

 but these fringes do not easily admit of being drawn out into a ribbon and 

 there is usually deficient light. The endeavor to distinguish the phases of 

 vibrating telephonic systems was partially successful, but the marked im- 

 portance of synchronism or resonance in these systems is the chief outcome 

 of the research. 



53. Telephonic apparatus. I began the work with two similar telephones, 

 as shown at t,t', in figure 66. Small mirrors were rigidly attached to the 

 centers of the diaphragms and each of the telephones secured on a standard 

 which admitted of adjustment around vertical and horizontal axes. The 

 intermittent current was supplied at a, b, by a small induction coil with a 

 rheostat in circuit. Four clamp-screws at c,d were available for putting the 

 telephone bobbins in series or in parallel. One telephone could be reversed 

 in action by the commutator K. White light L from a collimator was re- 

 flected or transmitted by the half-silvered mirrors M, M', N, N' of the inter- 

 ferometer and from m,m f on the telephones, as indicated by the arrows. 

 M' was on a micrometer with the screw on the direction n. To facilitate 

 the finding of the fringes one of the telephones, t', should also be on a microm- 

 eter with the screw normal to m'. The use of n requires special precau- 

 tions stated below. The fringes when found are observed by the vibration 

 telescope at T. It is sometimes difficult to catch the fringes even when using 

 the spectro-telescope, owing to the accentuated quiver of the system, and the 

 work is simplified by first supporting the diaphragm against vibration. 



The vibration telescope is shown in vertical section in figure 67, with the 

 ocular at E and the objective originally at e, the tube being supported on the 

 standard d and clamp cc, admitting of raising and lowering and slight 

 rotation around the horizontal axis 6. The objective A has been removed 

 and is now supported by a flat steel spring 5,5, in front of its former position. 

 Hence the ocular holder is a simple tube which can be thrust far inward and 

 clamped in any position by three screws at a. 



In order that the ocular may vibrate parallel to the fringes, and as these 

 appear in all angles of altitude, the special vibratory system f,g, k,s has been 

 devised. The rod reaching downward (about 10 to 15 cm. long) is attached 



1 Carnegie Inst. Wash. Pub. No. 149, Part III, 1914, pp. 208-213. 



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